The quality of power delivered by utility systems determines how well electrical and electronic equipment operates. Any disturbances to the power system can severely affect the equipment's performance. Power disturbances typically result from lightning, utility switching and utility outages. Such disturbances can also be created by the users of power through the switching of loads, ground faults, or abnormally high demand from heavy normal equipment operation. In each of these situations, the depletion of power through the line is severe enough to affect the operation of electrical equipment being used by other utility customers. In one example, the fluctuating load of a large welder in a mill producing wire mesh can cause lights and television sets to flicker for approximately 500 residential customers who received their power from the same feeder line used to supply power to the mill. Proposed solutions in this case included powering the equipment by a diesel generator during evening hours or installing a special electric utility line connected directly to the mill at a substantial cost. A more desirable solution to the problem of voltage stabilization is set forth in U.S. Pat. No. 5,194,803, entitled "Superconductive Voltage Stabilizer" and issued on Oct. 9, 1990, to Visser et al. The energy storage system is illustrated here in FIG. 1.
Referring to FIG. 1, a superconductive voltage stabilizer is generally designated by the numeral 10. Superconductive voltage stabilizer 10 includes an AC/DC converter 20, a superconducting coil 30, a voltage regulator 40 and an energy storage cell 50.
The superconductive voltage stabilizer 10 has an AC/DC converter for converting alternating current to direct current. Three-phase alternating current provided by an AC supply line is connected to AC input 22 of AC/DC converter 20. AC/DC converter 20 has a first DC terminal 24 and a second DC terminal 26. Once the alternating current input has been converted to direct current, a direct current output is available between the first and second DC terminals.
The direct current is then directed to a superconducting energy storage coil 30, through its connection to one of the DC terminals, which is used to store the energy created by the direct current and developed by AC/DC converter 20. Energy storage coil 30 stores energy depending on the control of voltage regulator 40. In its most basic embodiment, voltage regulator 40 comprises a current switch controller 42 and a current switch 44, for example, a gate turn off thyristor (GTO). AC/DC converter 20 controls the amount of current flowing through superconductive coil 30. Initially, current switch controller 42 activates current switch 44 so that a current path is created. When current switch 44 is activated, direct current can flow from first DC terminal 24, through coil 30, through current switch 44 and back through second DC terminal 26.
Once a sufficient amount of energy is stored in coil 30 and energy is required by the energy storage cell, voltage regulator 40 halts the current path through current switch 44 thereby directing the current through energy storage cell 50. Storage cell 50 comprises, in its most basic form, an energy storage capacitor 52. Energy storage cell 50 is connected in parallel with a load through a first output line 60 and a second output line 62. The voltage regulator 40, through the use of current switch controller 42, deactivates current switch 44 so that a new current path is created. Direct current can then flow from the first DC terminal 24, through energy storage coil 30, through a first input line 64 of energy storage cell 50, through energy storage cell 50, out through a second input line 66 of energy storage cell 50 and back through second DC terminal 26. Thus, energy is stored in energy storage cell 50 until the voltage across the cell 50 reaches a predetermined level. Once that level has been achieved, voltage regulator 40 directs the direct current away from energy storage cell 50 and back through the voltage regulator 40.
When energy storage cell 50 is sufficiently charged, the supply of energy in energy storage cell 50 can be delivered to power a load through a first output line 60 and a second output line 62 of energy storage cell 50. Output lines 60 and 62 cooperate to provide an output current path to the load. As the load draws energy away from energy storage cell 50, the voltage across cell 50, measured between the first input line 64 and the second input line 66 begins to drop. Once the voltage across cell 52 drops to a set level, it is sensed by voltage regulator 40. At that time, current switch controller 42 deactivates current switch 44, so that energy stored in superconducting coil 30 is delivered to energy storage cell 50 and the load. The delivery of stored energy continues until the voltage across energy storage cell 50 reaches a predetermined maximum value. At that point, voltage regulator 40 senses cell 50 is fully charged, and through current switch controller 42, activates current switch 44 so that current flows once again through current switch 44.
Current switch 44 can comprise an insulated gate bipolar transistor (IGBT) having a collector lead 45, an emitter lead 46, and a gate lead 47. Collector lead 45 is coupled to first input line 66, and gate lead 47 is coupled to current switch controller 42, which controls the conduction of IGBT 44 through gate lead 47. Various other devices can be used in place of IGBT 44 and can include gate-turn-off thyristors and silicon controlled rectifiers.
Thus, energy storage cell 50 supplies energy to the load. As energy is drawn from cell 50 by the load, voltage regulator 40 senses the voltage across the cell 50, and controls the amount of energy released from coil 30 to cell 50. A portion of the direct current stored in coil 30 is thereby delivered to energy storage cell 50 in accordance with the energy requirements of the load.
In such a voltage stabilizer, there is a need to protect the coil in the event of a "quench" condition. A quench condition occurs when a portion of the superconducting coil 30 goes from a superconducting state to a resistive state. In such instance, the portion of the coil 30 that is in a resistive state may be unable to dissipate the power flowing therethrough thus resulting in damage to the coil. A scheme for protecting the coil is therefore necessary.
One such protection scheme is illustrated in FIG. 2. The protection scheme includes a mechanical dump contactor which is connected in parallel with a dump resistor. When the coil is in a storage or charging mode of operation, the GTO is triggered to a conducting state by the gate driver which, in turn, receives commands from the voltage regulator. The dump contactor is placed in a normally closed condition thereby providing a continuous current path through the GTO or IGBT current switch, dump contactor, and coil.
The voltage across the coil will increase in the event of a quench condition. The voltage across the coil may thus be used to detect a quench condition and to generate a signal indicative of the quench condition. This generated signal is applied, for example, to a programmable logic controller (PLC) or other logic circuitry to generate a signal which is used to cause the mechanical dump contactor to go to an open condition. In this open condition, the current that would typically flow through the dump contactor is instead directed through the dump resistor. The dump resistor thus dissipates the coil energy thereby preventing damage to the coil. Additionally, a coil heater may be activated to cause even heating of the coil to bring all portions of the coil to a resistive state. This provides further protection of the coil since the heat dissipation in the coil becomes less centralized.
Although the illustrated protection scheme may function adequately in many situations, a scheme which provides a faster response time, additional protection, and allows discharge at high voltage levels is desirable.